Modulating O-H-based excited-state intramolecular proton transfer by alkyl-substitutions at various positions of 1-hydroxy-11H-benzo[b]fluoren-11-one

Abstract

The excited-state intramolecular proton transfer (ESIPT) reaction in 1-hydroxy-11H-benzo[b]fluoren-11-one (HBF) is found to be highly sensitive to the presence of specific substituents in positions C2, C4 and C8 with respect to the hydroxyl group, leading to different fluorescent behaviors of HBF with different substituents (J. Phys. Chem. C, 2018, 122, 21833). However, a systematic and comprehensive computational study of the influence of alkyl-substitutions at various positions of HBF on the intramolecular hydrogen bond strength, adsorption and fluorescence spectra and finally the proton transfer energy barriers is scare up to now. Therefore, in this work, the excited-state overall perspective of the proton transfer process of HBF and its seven tert-butyl substituted derivatives (Scheme 1) have been theoretically studied at TD-PBE0/TZVP theory level with the DFT and TDDFT methods. It is concluded that alkyl-substitution at positions C2 and C4 of the parent molecule should increase the strength of intramolecular hydrogen bonding O1-H⋯O, which decrease both the S1-state forward proton transfer energy barriers and the energy differences between forms S1-PT and S1 and eventually facilitate the ESIPT process, whereas alkyl-substitution at position C8 of the parent molecule should have slight opposite effect.